This invention relates to integral optical devices based on optical patterns created in porous glass bodies. In one embodiment of particular interest, the pattern is in the nature of an array of cylinders extending through the glass body. Each cylinder has a prescribed, radial, gradient, refractive index distribution. The distribution varies with radial distance, that is from the axial center line of the cylinder to the outside, in a parabolic manner, whereby lens-like properties are derived. In a specific improved construction, an array of cylindrical lens systems is capable of providing a single one-to-one erect conjugate image, as required for such purposes as photocopying.
Ser. No. 277,089, filed June 25, 1981 by two of us (Borrelli and Morse), now U.S. Pat. No. 4,403,031 discloses a method of producing an optical pattern in a porous glass by impregnating the glass with a photolyzable organometallic material and selectively photolyzing the impregnated glass. Among the optical patterns that may be created are gradient refractive index patterns, such as lenses, and optical density patterns.
The patent application defines a photolyzable organometallic as one which undergoes bond scission on exposure to light to produce a photolyzed metal-organic intermediate. This intermediate is preferably reactive, that is capable of reacting with the pore surfaces via hydroxyl groups to form coordination complexes or stronger bonds.
A porous glass is described as one incorporating a multiplicity of interconnected pores of submicron diameter into which the organometallic can be introduced as a liquid or a gas. Preferably, such glass is produced by phase separation and leaching techniques disclosed in U.S. Pat. Nos. 2,106,744, 2,215,036 and 2,221,709.
In accordance with the prior application disclosure, the organometallic material in unphotolyzed areas of the glass may be removed, as by washing or volatilizing. This avoids subsequent reactions in these areas which could blur or distort the pattern introduced. However, the nature of porous glass is such that it is strongly absorbent of moisture and foreign materials in the ambient atmosphere. This can be detrimental in optical devices. The porous glass may be consolidated to a nonporous state of course, but this is a high temperature step that may itself distort optical properties.
It is also frequently desirable to enhance the optical strength of elements created by the photolyzed organometallic process. This is particularly true in the case of lenses formed in arrays for imaging purposes. In such devices, the variable parameters are lens power, as determined by the index difference or gradient (n.sub.max -n.sub.o) created, and the lens system (glass) thickness.
The optical performance of a gradient index lens is fully developed in an article by F. Kapron in Journal of the Optical Society of America, 60, 1433-36 (1970). Further, an article by J. D. Rees in Applied Optics, 21, 1009 (1962) describes a lens array for one-to-one conjugate imaging.
An imaging device, capable of providing a one-to-one conjugate, erect image, might be provided if (1) adequate optical strength could be imparted to the individual lenses by enhancing the refractive index differential created in them, or if (2) a glass thickness greater than the normal 2 mm might be used. To this end, efforts were made to extend the exposure time, thereby enhancing the photolysis effect, and/or the depth of treatment in a porous glass body.
The basic aim then was to enhance the radial gradient index effect, and hence the lens power. It was also desired to extend this effect through the entire thickness of the glass body in as nearly uniform manner as possible. It was found, however, that extended exposure tended to create a severe axial gradient refractive index along the optical axis of the exposure radiation. This in turn caused distorted, non-symmetric imaging. Further, such ill effects increased sharply with exposure time.
It appeared critical then to limit exposure time, even though this failed to provide sufficient radial index change for the desired imaging effect. As a rule of thumb, glass thicknesses over about 2 mm were avoided, since the requisite exposure to achieve a reasonable power on the back surface was too great.
One solution to this problem is disclosed and claimed in companion application Ser. No. 520,458. As there described a plurality of samples or thickness not exceeding 2 mm are prepared. In each sample then, the axial gradient is controlled by controlled exposure. A plurality of samples are then aligned and bound in place to provide the required depth. However, this process does involve additional steps which would preferably be avoided in most instances.
Further, it would be highly desirable to be able to render opaque the matrix material intermediate lens systems. In an imaging device for example, this would minimize "cross-talk" and consequent image blurring. For present purposes, a lens system constitutes a cylindrical zone extending through a porous glass body and terminating on opposite faces of the body in planar surfaces that function as lens-like elements, the lens system having a prescribed, radial, gradient refractive index distribution created by photolysis of an organometallic compound in the pores.